This invention relates to the manufacture of electromagnetic components from amorphous metal discontinuous segments by compressing and molding said segments.
Various electrical components, such as motors and transformers are made up of laminations. By conventional practice, expensive carbide dies are used to punch laminations from steel strip. This process is time consuming and results in up to 50% scrap which is sold back to the steel mill at scrap prices and there are, in addition, handling and transportation costs.
To achieve a lower fabrication and assembly cost of electromagnetic devices, it would be highly desirable to be able to make part of, if not all, of the magnetic path from a moldable material. If acceptable magnetic properties could be achieved in such a moldable material, then the time-consuming and costly assembly operations of interleaving the core and coil of a transformer and the insertion of windings in the slots of motors would be largely eliminated. It is the provision of such a high quality magnetic moldable material to which this invention is directed.
It has been known for some time that the ideal shape of the individual magnetic particles making up this moldable magnetic material should be thin platelets. Such thin platelets when aligned with their longest dimension in the direction of the magnetic flux path will give the composite structure optimum magnetic properties. Platelets or oblate spheroids, as compared to prolate spheroids and to spheres, have the maximum area to transfer flux from one particle to the next yielding the lowest exciting field. With platelets aligned in the direction of flux, eddy current losses in the individual particles will be minimized.
Iron powders in the form of carbonyl iron have been used in composites. When the individual particles are coated with insulating material to reduce eddy current losses of the composite, permeabilities in the 10-20 range are obtained because of the high excitation required to drive the flux through the space between the spheres. Use of this type of material has been limited primarily to radio frequencies because of this low permeability and high cost.
Flakes of iron have been made by rolling powder. Properties of composites made from these flakes are substantially better than composites made from powder but still have relatively high hysteresis losses. These high hysteresis losses are attributed to the random directions of the crystals in the individual flakes, strains in the flakes from pressing as well as boundary impediments to domain wall motion.
Because the crystalline axes are randomly oriented, the anisotropy associated with these axes will be random and yield low permeabilities. Inclusions at the grain boundaries and the high internal stresses inhibit the motion of domain walls. This is a major cause of high losses. Spherical powders suffer from these high hysteresis losses in addition to the high exciting field requirement.
Amorphous magnetic metals, unlike normal crystalline magnetic metals, have no long range atomic order in their structure. Therefore, the directionality of properties such as magnetization normally associated with crystal anisotropy is absent. Also, unlike normal metals, amorphous metals are extremely homogenous, being devoid of inclusions and structural defects. These two characteristics--magnetic isotropy and structural homogeneity--give amorphous metals unusually good dc magnetic properties. The magnetic isotropy leads to extremely low field requirements for saturation, and the structural homogeneity allows the magnetization to reverse with extremely low fields (i.e., a low coercive force). These two features combined with the high resistivity (15 times that of common iron) and lamination thinness provide a material with the lowest ac losses of any known high magnetic saturation material.
Amorphous structures can be obtained by several techniques. Electroplating, vapor deposition, and sputtering are all techniques where the material is deposited on an atom by atom basis. Under specific conditions, the atoms are frozen in place on contact and do not have a chance to move to the lower energy positions of the thermal crystal lattice sites. The resulting structure is an amorphous, noncrystalline glassy one. These methods, however, are not economical for producing large commercial quantities.
The other method for producing amorphous structures in metals is by cooling rapidly from the liquid melt. Two conditions must be met to achieve the amorphous structure by this method. First, the composition must be selected to have a high glass transition temperature, T.sub.g, and a low melting temperature, T.sub.m. Specifically, the T.sub.g /T.sub.m ratio should be as large as possible. Second, the liquid must be cooled as rapidly as possible from above T.sub.m to below the T.sub.g. In practice, it is found that to produce metallic glasses, the cooling rate must be of the order of a million degrees centigrade per second. Even at these high rates, only special compositions can be made amorphous. Typically, "glass forming" atoms such as the metalloids, phosphorous, boron, silicon, and carbon are required additions to the metal alloy, usually in the 10 to 25 atomic percent range.
In machines, such as motors and transformers, there are design requirements on the geometry of the magnetic material. These requirements depend on the properties of the material and the physical structure of the device. Ideally, the material should be continuous along the flux path to form a completely closed magnetic circuit. This would provide the highest permeability possible for the circuit and the lowest excitation current requirements. This geometry is not possible with normal laminated electrical steel because the assembly requirements necessitate cutting the magnetic material. For example, in transformers a complex interleaved joint is fabricated to partially offset the negative effect on the permeability from this cutting. Another special geometric requirement on an ac machine is that the magnetic material be thin in a plane perpendicular to the flux direction. This is essential to minimize the eddy current losses. However, with decreasing lamination thickness more laminations are needed so the punching time and assembly costs increase.